Pham, Gia Khanh; Keppeler, Roman; Lampenscherf, Stefan; Goldberg, Vitali. (2021). Fast Characterization Method for Thermal Interface Materials (TIMs) for Electronic Applications. European Congress and Exhibition on Advanced Materials and Processes (2021, Vitruell).


Thermal Interface Materials (TIMs) are utilized as contact material between an electronic component (e.g. power electronics of e-cars) and the heat sink. The considerably increased power requirements and the continuing trend towards miniaturization in electronic devices lead to an increased performance and desire a suitable characterization to understand the behavior of Thermal Interface Materials. As a key component in the thermal management of electronic components, TIMs should have high electrical insulation properties (dielectric strength of about 5 kV) and high thermal conductivity to prevent an overheating in electronic components. With so many TIMs available on the market, it is getting more and more difficult to make the right decision when selecting a best matched TIM for the specific application. Parameters such as thermal resistance, thermal conductivity, electrical insulation, contact pressure dependency and price play an important role in TIMs selection. Data sheet specifications for thermal resistance are mostly determined by the ASTM D5470 method. In this case, TIMs characterization is performed under highly favorable conditions (polished surfaces, excessive pressure conditions). However, these values differ when TIMs are used for electronic applications. This can lead to incorrect thermal management design and thus damage the electronic component. In this study, a fast method for characterization of the thermal resistance of TIMs under realistic operating conditions was developed. A 3 D printed test rig was built up to measure the thermal resistance of TIM samples (pads and foils) at different contact pressures, volume and heat flow rates. The heat source was uniform distributed as well as only locally applied which is typical for practical applications. For the study four commercially available TIMs were selected and characterized: Elastomer, silicone, phase change material (PCM) and graphite foil with support layers. The measured temperatures were used to calculate the thermal resistance of the respective TIMs. The results show a strong dependence of the thermal resistance on the sample thickness and the contact pressure for all samples. The PCM sample showed the lowest thermal resistance and elastomer sample showed the highest. Applying a non-uniform heat source, the graphite foil could not demonstrate its benefit in terms of in-plane heat distribution, like a heat spreader. A comparison of the measured values with the manufacturer's data showed a deviation between ASTM method and typical operation conditions as investigated here. Using this method TIM materials and systems can be fast characterized and allows a rapid indication of the applicability of the desired TIM under relevant operation conditions.  


Keppeler, Roman; Tangermann, Eike; Allauddin, Usman; Pfitzner, Michael. (2013). LES of low to high turbulent combustion in an elevated pressure environment. Flow, Turbulence and Combustion, Vol.92, No.3, S.767-802.


A subgrid scale flame surface density combustion model for the Large Eddy Simulation (LES) of premixed combustion is derived and validated. The model is based on fractal characteristics of the flame surface, assuming a self similar wrinkling of the flame between smallest and largest wrinkling length scales. Experimental and direct numerical simulation databases as well as theoretical models are used to derive a model for the fractal parameters, namely the cut-off lengths and the fractal dimension suitable in the LES context. The combustion model is designed with the intent to simulate low as well as high Reynolds number premixed turbulent flame propagation and with a focus on correct scaling with pressure. The combustion model is validated by simulations of turbulent Bunsen flames with methane and propane fuel at pressure levels between 0.1 MPa and 2 MPa and at turbulence levels of 0 < u‘/s0L < 11, conditions typical for spark ignition engines. The predicted turbulent flame speed is in a very good agreement with the experimental data and a smooth transition from resolved flame wrinkling to fully modelled, nearly subgrid-only wrinkling is realized. Evaluating the influence of mesh resolution shows a predicted mean flame surface and turbulent flame speed independent of mesh resolution for cases with 9–86 % resolved flame surface. Additional simulations of a highly turbulent jet flame at 0.1 MPa and 0.5 MPa and the comparison with experimental data in terms of flame shape, velocity field and turbulent fluctuations validates the model also at conditions typical for gas turbines.




Keppeler, Roman; Pfitzner, Michael; Chong, Luis Tay Wo; Komarek, Thomas; Polifke, Wolfgang. (2012). Including heat loss and quenching effects in algebraic models for Large Eddy Simulation of premixed combustion. ASME Turbo Expo (2012, Kopenhagen).


The world is facing critical energy concern, in view of depleting fossil fuel reserves and increasing environment pollution. Biodiesel is a promising substitute that can reduce our dependence on fossil fuel. The application of biodiesel in microturbines is a new approach in the power generation industry. There are limited data at present on the potential of biodiesel in non-transportation sectors, where substantial energy and environment benefits exist. The objective of this paper is to study the feasibility of biodiesel to substitute conventional microtubine fuel through an atomization characteristics study in a fuel spray atomizer. Various blends of biodiesel derived from waste vegetable oil were tested experimentally in a fuel spray atomizer, to obtain main atomization characteristics such as Sauter Mean Diameter, spray angle and spray tip penetration. From the fuel properties analysis, it was found that the higher content of biodiesel gives higher viscosity, density and surface tension of the fuel. This will result in larger droplet size and longer spray length but smaller spray angle and spray width with clearer vortex shape of spray pattern in the atomization test experiment. Meanwhile, higher injection pressure during atomization testing process tend to break up fuel particles into smaller partition, which will subsequently produce larger spray angle and spray width but shorter spray length with denser spray pattern. The results of the experiment proved that biodiesel is a viable alternative fuel to gas turbine applications.




Keppeler, Roman; Kranawetvogl, J.; Jarczyk, Maria-Magdalena; Pfitzner, Michael. (2011). Large Eddy Simulationen von Rohr- Kanal- und Freistrahlströmung mit OpenFOAM. Institut für Thermodynamik Fakultät für Luft und Raumfahrttechnik Universität der Bundeswehr München.

Keppeler, Roman; Tangermann, Eike; Pfitzner, Michael. (2011). Large Eddy Simulation of a Premixed Turbulent Methane and Propane Bunsen Flame at Elevated Pressure. Highly Resolved Combustion Diagnostics Numerics and Experiments (2011, Rouen).

Keppeler, Roman; Tangermann, Eike; Pfitzner, Michael. (2011). Entwicklung und Validierung eines Grobstruktursimulations Modells für vorgemischte Verbrennung. Deutscher Flammentag (25., 2011, Karlsruhe).

Keppeler, Roman; Tangermann, Eike; Pfitzner, Michael. (2011). Enhancement of a new Large Eddy Simulation combustion model to simulate high pressure and low Reynolds number flows. European Combustion Meeting (5., 2011, Cardiff, UK).

Keppeler, Roman; Pfitzner, Michael; Chong, Luis; Zellhuber, M.; Polifke, Wolfgang. (2011). Accounting for the Combined Effects of Strain and Heat Loss on Premixed Turbulent Combustion in an LES Model. International Conference on Numerical Combustion (13., 2011, Corfu).


Keppeler, Roman; Tangermann, Eike; Pfitzner, Michael. (2010). Large Eddy Simulation of premixed turbulent Bunsen flames using a new fractal based combustion model. Euromech Fluid Mechanics Conference (8., 2010, Bad Reichenhall).